The mechanisms by which cariogenic and noncariogenic Streptococci and certain members of the genus Actinomyces regulate the distribution of substrate carbon among multiple catabolic pathways is under investigation. Several enzymes, which function in the constitutive pathways that lead to lactic acid, have been identified as specific control sites. These include glyceraldehyde-3-phosphate dehydrogenase, pyruvate kinase, and lactate dehydrogenase. In Actinomyces viscosus, two enzymes involved in ammonia assimilation are also under investigation. The means by which the catalytic function of these enzymes is modulated by their interaction with specific low molecular weight ligands is being studied. These comparative studies with cariogenic and noncariogenic streptococci are designed to gain insight into a possible biochemical basis for pathogenicity and ultimately to formulate a rational approach for its control.